Field of the Invention
Example embodiments relate generally to a system and method for generating restricted sets of cyclic shifts for long-term evolution (LTE) physical random access channel (PRACH) preambles in order to control telecommunication traffic on cells that serve user equipments (UEs) traveling at a high rate of speed.
Related Art
FIG. 1 illustrates a conventional 3rd Generation Partnership Project Long-Term Evolution (3GPP LTE) network 10. The network 10 includes an Internet Protocol (IP) Connectivity Access Network (IP-CAN) 100 and an IP Packet Data Network (IP-PDN) 1001. The IP-CAN 100 generally includes: a serving gateway (SGW) 101; a packet data network (PDN) gateway (PGW) 103; a policy and charging rules function (PCRF) 106; a network management function (NMF) 107; a mobility management entity (MME) 108 and E-UTRAN Node B (eNB) 105 (i.e., base station, for the purposes herein the terms base station and eNB are used interchangeably). Although not shown, the IP-PDN 1001 portion of the EPS may include application or proxy servers, media servers, email servers, etc.
Within the IP-CAN 100, the eNB 105 is part of what is referred to as an Evolved Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (EUTRAN), and the portion of the IP-CAN 100 including the SGW 101, the PGW 103, the PCRF 106, the NMF 107 and the MME 108 is referred to as an Evolved Packet Core (EPC). Although only a single eNB 105 is shown in FIG. 1, it should be understood that the EUTRAN may include any number of eNBs. Similarly, although only a single SGW, PGW and MME are shown in FIG. 1, it should be understood that the EPC may include any number of these core network elements.
The eNB 105 provides wireless resources and radio coverage for one or more user equipments (UEs) 110. That is to say, any number of UEs 110 may be connected (or attached) to the eNB 105. The eNB 105 is operatively coupled to the SGW 101, the NMF 107, and the MME 108.
The SGW 101 routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNB handovers of UEs. The SGW 101 also acts as the anchor for mobility between 3rd Generation Partnership Project Long-Term Evolution (3GPP LTE) and other 3GPP technologies. For idle UEs 110, the SGW 101 terminates the downlink data path and triggers paging when downlink data arrives for UEs 110.
The PGW 103 provides connectivity between UE 110 and the external packet data networks (e.g., the IP-PDN) by being the point of entry/exit of traffic for the UE 110. As is known, a given UE 110 may have simultaneous connectivity with more than one PGW 103 for accessing multiple PDNs.
The PGW 103 also performs policy enforcement, packet filtering for UEs 110, charging support, lawful interception and packet screening, each of which are well-known functions. The PGW 103 also acts as the anchor for mobility upon SGW relocation during handovers within LTE network, as well as between 3GPP and non-3GPP technologies, such as Worldwide Interoperability for Microwave Access (WiMAX) and 3rd Generation Partnership Project 2 (3GPP2 (code division multiple access (CDMA) 1X and Enhanced Voice Data Optimized (EvDO)).
Still referring to FIG. 1, eNB 105 is also operatively coupled to the MME 108. The MME 108 is the control-node for the EUTRAN, and is responsible for idle mode UE 110 paging and tagging procedures including retransmissions. The MME 108 is also responsible for choosing a particular SGW for a UE during initial attachment of the UE to the network, and during intra-LTE handover involving Core Network (CN) node relocation. The MME 108 authenticates UEs 110 by interacting with a Home Subscriber Server (HSS), which is not shown in FIG. 1.
Non Access Stratum (NAS) signaling terminates at the MME 108, and is responsible for generation and allocation of temporary identities for UEs 110. The MME 108 also checks the authorization of a UE 110 to camp on a service provider's Public Land Mobile Network (PLMN), and enforces UE 110 roaming restrictions. The MME 108 is the termination point in the network for ciphering/integrity protection for NAS signaling, and handles security key management.
The MME 108 also provides control plane functionality for mobility between LTE and 2G/3G access networks with an S3 type of interface from the SGSN (not shown) terminating at the MME 108.
The Policy and Charging Rules Function (PCRF) 106 is the entity that makes policy decisions and sets charging rules. It has access to subscriber databases and plays a role in the 3GPP architecture as specified in 3GPP TS 23.203 “Policy and Charging Control Architecture.” The network management function (NMF) 107 is the entity that controls operations of the Radio Access Network.
The IP-PDN 1001 network may include an application function (AF) 109. The Application Function (AF) 109 is an entity that is application aware and is an ultimate receiver of exported eNB data that may be used to more effectively deliver content to the UE 110 to improve and/or optimize the network 10. AF 109 may alternatively or additionally be positioned inside the UE 110.
FIG. 2 illustrates a conventional E-UTRAN Node B (eNB) 105. The eNB 105 includes: a memory 240; a processor 220; a scheduler 210; wireless communication interfaces 260; Radio Link Control (RLC) and Medium Access Control (MAC) layer control 230 for each bearer; and a backhaul interface 235. The RLC and MAC layer control 230 is responsible for RLC and MAC layer protocol signaling, as defined by the 3GPP standards. The processor or processing circuit 220 controls the function of eNB 105 (as described herein), and is operatively coupled to the memory 240 and the communication interfaces 260. While only one processor 220 is shown in FIG. 2, it should be understood that multiple processors may be included in a typical eNB 105. The functions performed by the processor may be implemented using hardware. Such hardware may include one or more Central Processing Units (CPUs), digital signal processors (DSPs), application-specific-integrated-circuits, field programmable gate arrays (FPGAs) computers or the like. The term processor, used throughout this document, may refer to any of these example implementations, though the term is not limited to these examples. With a Virtual Radio Access Network (VRAN) architecture various functions eNB components may be distributed across multiple processing circuits and multiple physical nodes within VRAN cloud.
The eNB 105 may include one or more cells or sectors serving UEs 110 within individual geometric coverage sector areas. Each cell individually may contain elements depicted in FIG. 2. Throughout this document the terms eNB, cell or sector shall be used interchangeably.
Still referring to FIG. 2, the communication interfaces 260 include various interfaces including one or more transmitters/receivers connected to one or more antennas to wirelessly transmit/receive control and data signals to/from UEs 110, or via a control plane. Backhaul interface 235 is the portion of eNB 105 that interfaces with SGW 101, MME 108, other eNBs, or interface to other EPC network elements and/or RAN elements within IP-CAN 100. The scheduler 210 schedules control and data communications that are to be transmitted and received by the eNB 105 to and from UEs 110. The memory 240 may buffer and store data that is being processed at eNB 105, transmitted and received to and from eNB 105.
Every Transmission Time Interval (TTI), typically equal to 1 millisecond, the scheduler 210 may allocate a certain number of Physical Resource Blocks (PRBs) to different bearers carrying data over the wireless link in the downlink direction (i.e., transmitting buffered data from eNB 105 to UE 110) and uplink direction (i.e., receiving data at eNB 105 from UE 110, which is received over backhaul 235). A “bearer” may be understood to be a virtual link, channel, or data flow used to exchange information for one or more applications on the UE 110. The scheduler 210 may determine Modulation and Coding Schema (MCS) that may define how many bits of information may be packed into the allocated number of PRBs.
Scheduler 210 may make PRB allocation decisions based upon a Quality of Service (QoS) Class Identifier (QCI), which represents traffic priority hierarchy. There are nine QCI classes currently defined in LTE, with 1 representing highest priority and 9 representing the lowest priority. QCIs 1 to 4 are reserved for Guaranteed Bitrate (GBR) classes for which the scheduler maintains certain specific data flow QoS characteristics. QCIs 5 to 9 are reserved for various categories of Best Effort traffic.
Conventionally, cyclic shift distortion due to uplink frequency offset may impact a received power delay profile during long-term evolution (LTE) physical random access channel (PRACH) detection between a UE 110 and an eNB 105. This distortion may create false alarm peaks where a relative amplitude to the correct peak depends on a frequency offset. For example, when an uplink frequency offset exceeds half of the PRACH subcarrier spacing (i.e., +/−0.625 KHz), the false alarm peaks may exceed the correct peak. In order to retain an acceptable false alarm rate and maintain a high detection performance for a high-speed UE 110 with large uplink frequency offsets, LTE standard 3GPP TS 36.211, ‘E-UTRA: Physical Channels and Modulation,’ V12.5.0 (2015-03), specifies a cyclic shift restricted set that can mask some cyclic shift positions in a Zadoff-Chu (ZC) root sequence. The cyclic shift restricted set may be effective when uplink frequency offset is within the PRACH subcarrier spacing (i.e., +/−1.25 KHz).
In order to access the LTE network, a UE 110 needs first to send a physical layer random access preamble through LTE physical random access channel (PRACH) to the eNB 105. The set of preamble sequences the UE 110 is allowed to use is generally configured by the network.
Conventionally, each physical layer random access preamble contains a cyclic-shifted Zadoff-Chu sequence that is generated from a root Zadoff-Chu sequence. As defined in TS 36.211, the uth root Zadoff-Chu sequence is expressed as follows.
                                                        x              u                        ⁡                          (              n              )                                =                      e                                          -                j                            ⁢                                                π                  ⁢                                                                          ⁢                                      un                    ⁡                                          (                                              n                        +                        1                                            )                                                                                        N                  ZC                                                                    ,                  0          ≤          n          ≤                                    N              ZC                        -            1                                              Equation        ⁢                                  ⁢        1            
Where the NZC is conventionally called the length of the Zadoff-Chu sequence (NZC is 839 for Preamble format 0 to Preamble format 3).
As defined in TS 36.211, from the uth root Zadoff-Chu sequence, random access preambles are defined by cyclic shifts Cv according to the following.xu,v(n)=xu((n+Cv)mod NZC)  Equation 2
Where the cyclic shift Cv for a unrestricted set may be given by the following equation.
                              C          v                =                  {                                                                      0                  ,                                                            N                      CS                                        =                    0                                                                                                                                            vN                    cs                                    (                                                            v                      =                      0                                        ,                    1                    ,                    …                    ⁢                                                                                  ,                                                                  ⌊                                                                              N                            ZC                                                    /                                                      N                            CS                                                                          ⌋                                            -                      1                                        ,                                                                  N                        CS                                            ≠                      0                                                                                                                              Equation        ⁢                                  ⁢        3            
The parameter NCS is a configured cyclic shift value used for random access preamble generation.
In order to determine which random access preamble is sent by the UE 110, the eNB 105 needs to correctly determine which Zadoff-Chu sequence is included in the received random access preamble. Thus, the eNB 105 needs to determine correctly the cyclic shifts Cv of the Zadoff-Chu sequence used for the random access preamble.
For a UE 110 travelling at high speed, there may be a large uplink frequency offset in the random access preamble received by the eNB 105 due to the impact of Doppler shift. The uplink frequency offset caused by the high speed may cause the distortion of the received PRACH preamble. For example, when the uplink frequency offset is near the PRACH subcarrier spacing), a transmitted preamble using a Zadoff-Chu sequence with the cyclic shift Cv may be falsely detected as a preamble using another Zadoff-Chu sequence with the cyclic shifts Cv±du where du is a cyclic shift associated with the root Zadoff-Chu sequence index u, which will be further discussed in Equation 7. For preamble formats 0-3, the PRACH subcarrier spacing is conventionally defined as 1.25 KHz (see Table 5.7.3-1 of 3GPP TS 36.211, ‘E-UTRA: Physical Channels and Modulation’). For a preamble format of 4, the PRACH subcarrier spacing is conventionally defined as 7.5 KHz (see Table 5.7.3-1 of 3GPP TS 36.211). Because it is highly unlikely that the uplink Doppler frequency will reach 7.5 KHz, it is presumed for purposes of this document that the PRACH preamble formats 0-3 will be of primary concern.
In order to avoid the false detection problem due to uplink frequency offset, LTE standard 3GPP TS 36.211 (‘E-UTRA: Physical Channels and Modulation’) defines a conventional restricted set of cyclic shifts for cells serving high speed UEs 110. The conventional restricted set of cyclic shifts work by removing the cyclic shifts, which may cause detection ambiguity, from the unrestricted set of cyclic shifts (see Equation 3). For example, if a cyclic shift Cv is included in the restricted set, then the cyclic shifts Cv±du are purposely excluded from the restricted set. In this way, if the eNB 105 detects a preamble with cyclic shifts Cv±du, the eNB 105 understands the reason for receiving preamble with cyclic shifts Cv±du is due to Doppler impact, since cyclic shifts Cv±du are excluded from the restricted set. In this way, the eNB 105 may correctly identify that the preamble is transmitted with cyclic shift Cv. The restricted sets of cyclic shifts in TS 36.211 are designed to cover the frequency offset up to RACH subcarrier spacing, i.e., 1.25 KHz.
In many countries, high speed trains (HSTs) are now travelling faster than 300 km/h. For instance, in Japan HSTs may travel at 320 km/h, in Germany HSTs may travel at 330 km/h, in Italy HSTs may travel at 400 km/h, and in China HSTs may travel at 430 km/h. For trains traveling over 300 km/h, the uplink frequency offset may far exceed the PRACH subcarrier spacing 1.25 KHz. For example, for a carrier frequency of 2.6 GHz, and a train speed of 430 km/h, the uplink frequency offset caused by Doppler shift alone will be 2.070 KHz. Thus, the conventional cyclic shift restricted set defined in 3GPP TS 36.211 may no longer be suitable for these HST scenarios, and thus a new cyclic shift restricted set needs to be introduced to support uplink frequency offset far exceeding the PRACH subcarrier spacing of 1.25 KHz.